1. Field of the Invention
This invention relates generally to a digital limiter, and more particularly to a digital limiter suitable for use in limiting the amplitude of a video signal with its aperture corrected.
2. Description of the Prior Art
FIG. 1 shows a conventional processing circuit for a video signal supplied, for example, from a video camera. In FIG. 1, an input video signal Si is converted by an A/D converter 31 to be an 8-bit digital signal per one sample. The 8-bit straight binary video signal from the A/D converter 31 is supplied to an adder circuit 32 as well as an aperture correction signal generating circuit 33. An 8-bit offset binary aperture correction signal S.sub.AP therefrom is supplied to the adder circuit 32 wherein the aperture correction signal S.sub.AP is added to the video signal so as to produce a 9-bit video signal with its edge portions emphasized. In this event, a range of the 8-bit data, for example, from "00" to "FF" (hexadecimal) is assumed to correspond to a range of the 9-bit data from "80" to "180" (hexadecimal). However, the 9-bit video signal produced by adding the aperture correction signal S.sub.AP to the 8-bit video signal may be out of the range of "80" to "180". Further, following circuits are designed to process 8-bit data which is equal to the original digital video signal, so that the excess of the data produced by the addition has to be deleted.
For this purpose, the 9-bit video signal from the adder circuit 32 is supplied to a digital limiter 34 wherein the data portion out of the range of "80" to "180" is amplitude-limited so that 8-bit data within the range of "00" to "FF" can be derived therefrom.
The 8-bit video signal from the digital limiter 34 is supplied through, for example, a .gamma.-correction circuit 35 processing 8 bit data to a D/A converter 36 from which an analog video signal So is derived.
The digital limiter 34 of FIG. 1 may be a circuit which performs such limiting operations that data above "180" (hexadecimal) is simply replaced by a maximal 8-bit data i.e. "11111111" and data below "80" is replaced by a minimal 8-bit data "00000000". However, if the signal thus limited is subjected to a digital-to-analog conversion, high frequency components are produced in the outputted analog signal. Further, if an interpolation circuit having zero dimension hold effect is used at the rear stage of the D/A converter, due to the phase rotation caused by the group delay characteristic of the interpolation filter there may be a concern that overshoot or the like will be produced in the output signal if a signal having certain frequency components is supplied thereto.
There may also be considered, as the digital limiter 34 of FIG. 1, an arrangement formed of conversion tables stored, for example, in ROM or the like and having an input/output characteristic (limiter characteristic) as shown in FIG. 2. As can be seen from FIG. 2, the slope of the curve near the upper limit value "FF" in the white direction and the lower limit value "00" in the black direction changes gradually so as to suppress the overshoot and undershoot, which is a defect of the above-mentioned digital limiter, caused by the interpolation filter after digital-to-analog conversion. Thus, a video signal is subjected to a so-called soft limit operation.
However, by the above described arrangement for effecting the soft limit operation, data within the upper and lower limits but near the upper and lower limits are compressed, so that the linearity of the characteristic curve is compromised.